Gasses
dissolve in liquids. The gas molecules occupy space between the liquid
molecules.

But, when all of the space has been used, the liquid becomes
saturated. No more gas can be dissolved once a liquid becomes saturated.
Air contains about 21 percent oxygen, and some oxygen dissolves in wine
anytime wine is exposed to air.

Depending on exposed area,
temperature, etc., oxygen dissolves quickly until wine becomes
saturated. The dissolved oxygen then reacts with some of the other
materials in the wine, and the dissolved oxygen disappears from the
wine. However, many wine oxidation reactions take place slowly and
considerable time may be required for the dissolved oxygen to completely
disappear from wine. Depending on storage conditions, several days to
several weeks may be required before all of the dissolved oxygen is
gone.

Several winemaking operations depend
on how gasses dissolve and saturate liquids, so understanding dissolved gases
is of value to winemakers. Three examples are discussed below.

Oxygen for Fermentation

Yeast needs oxygen to multiply
and produce new cells, and fermentations can be sluggish or even stick when
not enough oxygen is present in the juice. Popular strains of wine yeast
need from 5 to 15 milligrams of oxygen per liter of juice to produce the
large cell population needed for a vigorous fermentation. (Sometimes, just
racking and splashing the juice to introduce a new supply of oxygen can
restart stuck fermentations).

Grape juice becomes saturated with
oxygen during normal crush operations, and saturated juice contains roughly
10 milligrams of dissolved oxygen per liter. If enough sulfur dioxide
is added to deactivate oxidative enzymes, this dissolved oxygen will remain
in the juice for some time. Little extra oxygen is needed by the yeast
to complete fermentation because much of the needed oxygen is already in the
saturated juice. This is why fermentations can be done in large, closed
tanks even though the yeast requires oxygen early in the fermentation
process.

Removing Hydrogen Sulfide

Professional winemakers prefer to
use copper to remove the stench of hydrogen sulfide from wine. On the
other hand, most home winemakers prefer to splash wine vigorously to remove
hydrogen sulfide gas. The major disadvantages to splashing are the
possibility of oxidizing the wine and the possibility of oxidizing mercaptan into disulfide. However, the amount of
oxygen entering the wine can be reduced considerably if the splashing is done
properly.

Splashed wine quickly becomes
saturated with oxygen, and once saturated, little more oxygen can enter the
wine. The saturated wine can then be splashed for an extended time to
remove the hydrogen sulfide gas. Because of the saturation
effect, one large exposure to air produces much less oxidation than several
splashing treatments applied over an extended time.

Oops

Novice winemakers often panic when
they discover a bung on the floor because they are afraid the wine in the open
barrel will be oxidized. Depending on storage conditions, wine in an
open barrel becomes saturated with oxygen quickly, but once the wine is
saturated little more oxygen can enter. Some of the dissolved oxygen
reacts slowly with sulfur dioxide or other materials in the wine. The
dissolved oxygen disappears slowly, so the wine in the open barrel remains
saturated or nearly saturated.

Once the bung is replaced, sulfur
dioxide continues to scavenge oxygen. Consequently, a barrel of wine
may be open to the air for a day or so, but the saturation effect often
prevents excessive wine damage. Of course, catastrophic oxidation can
occur if the sulfur dioxide content of the wine is low or a barrel remains
open for an extended time.